This invention relates to a method of manufacturing a heater of an indirectly-heated type cathode structure of an electronic tube, and a heater manufactured by said method.
In a conventional heater of an indirectly-heated type cathode structure, the heater comprises a main portion formed by helically winding a heater wire and then shaping into a desired configuration, leg portions connected to the opposite ends of the main portion and an electric insulator such as alumina coating the main portion and a part of the leg portions so that the heater has a large thermal capacity throughout these portions. Thus, the free, terminal or end parts of the legs are bare and connected to bare lead wires for connection with a heater source. Since the thermal capacity of bare leg terminal parts are smaller than that of the portions covered with the electric insulator, the bare terminal parts of legs are rapidly heated immediately after starting and before an electronic tube incorporated with the indirectly-heated type cathode electrode reaches normal operating condition, resulting in frequent breaking of the heater wire. Moreover, since the portions covered with the insulator and having a relatively large weight are supported by the bare leg terminal parts, where the length of the heater is long and where the heating wire is thin, when subjected to external shocks and vibrations, not only the heating wire breaks but also a portion of the insulator scatters, turning into foreign material to the tube, thereby causing failure of the tube.
To solve this problem, it has been proposed to weld lead wires to a coil heater and then to coat the bare ends of the legs with such insulator as alumina. This method, however, cannot positively prevent peeling off of the insulator because the insulator coated after welding lead wires cannot be sintered at a high temperature and hence bonding of alumina is poor.
According to another solution, a mandrel made of metal having high melting temperature and an inner diameter substantially equal to the diameter of the coil is fitted into the leg ends until the top of the mandrel adjoins the insulator coating. Application of the mandrel to a coil heater made of a fine heating wire requires careful hand working.
According to still another method, cylindrical metal sheaths are fitted over the ends of the bare legs. Although this method is advantageous where the legs are straight, in case where the heater is formed by helically winding a thin heating wire, it often happens that the coiled leg ends are crushed when applying the sheath thereover, causing the heating wire to be broken. This method also requires hand working.